1) Field of the Invention
The field of the invention generally relates to pulp processing and, more specifically, to a method and system for efficient production of different types of pulp using a kraft cooking process.
2) Background
Pulp created from organic materials, such as wood chips or other vegetation, can be processed into a relatively low grade cellulose product for uses such as paper, or into a relatively high grade cellulose product for making various synthetic fabrics or products. For example, high grade pulps can potentially be used to make rayon and other synthetics and textiles, or can be used to make cellulose acetate or cellulose esters which have a variety of commercial uses. This higher grade of pulp, which is available in different levels of purity, is commonly referred to as dissolving pulp, and is generally much more expensive than paper grade pulp.
A number of chemical and mechanical methods exist for processing organic materials in order to manufacture pulp products such as paper. Some of the basic steps include preparing the raw material (e.g., debarking and chipping), separating the wood fibers by mechanical or chemical means (e.g., grinding, refining or cooking) to separate the lignin and extractives from cellulose of the wood fibers, removing coloring agents by bleaching, and forming the resulting processed pulp into paper or other products. In addition to and in connection with pulp and paper manufacturing, pulp mills also typically have facilities to produce and reclaim chemical agents, collect and process by-products to produce energy, and remove and treat wastes to minimize environmental impact.
A well known process for manufacturing pulp is known as the kraft process, which has been around for many decades. In a typical kraft process, organic materials are treated with chemicals and heat in order to liberate lignins and purify the cellulose within the organic materials. The organic material may be treated with an aqueous mixture of sodium hydroxide and sodium sulfide, known as white liquor. The treatment breaks the linkage between lignin and cellulose, and degrades most of lignin and a portion of hemicellulose macromolecules into fragments that are soluble in strongly basic solutions. This process of liberating lignin from surrounding cellulose is known as delignification. The soluble portion is thereafter separated from the cellulose pulp.
When making dissolving grade pulp, a goal is to achieve a high cellulose purity or quality. Pulp quality can be evaluated by several parameters. For example, the percentage of alpha cellulose content expresses the relative purity of the processed pulp. The alpha cellulose content can be estimated and calculated based on the pulp solubility (e.g., S10 and S18 factors). The degrees of delignification and cellulose degradation are measured by Kappa Number (“KN”) and pulp viscosity respectively. A higher pulp viscosity indicates longer cellulose chain length and lesser degradation.
When making dissolving pulp, it is common to perform pre-hydrolysis and neutralization steps on the pulp prior to cooking. Pre-hydrolysis is generally performed to remove hemicellulose, to lesser extent lignins. Pre-hydrolysis is generally performed utilizing hot water, steam, acid (usually sulphuric) or any combination of those. After pre-hydrolysis, the organic material is neutralized with a neutralization liquor (typically alkali media) such as caustic, white liquor, weak black liquor or any combination of those. After neutralization, the organic material is cooked in a digester along with various cooking liquors in order to further dissolve the hemicellulose and lignins. The resulting cooked pulp, known as brownstock, may be collected, washed, and bleached by downstream processes in order to produce pulp of desired characteristics.
When making paper grade pulp, it is not necessary to carry out the same type of pre-hydrolysis and neutralization as carried when making dissolving pulp. The cooking stage for paper grade pulp is generally similar to the cooking process for making dissolving grade pulp. The process of making paper grade pulp generally has a higher yield than that used for dissolving grade pulp, because when in the absence of pre-hydrolysis and neutralization significantly less hemicellulose is removed.
There are two basic types of digesters used for pulp production. The first type is referred to as a batch digester, which is a type of vertical pressure vessel generally used to perform sequential processing steps on the pulp. When making dissolving pulp, the same digester or vertical pressure vessel is first used for pre-hydrolysis and neutralization, and then for kraft cooking. The second type of digester is referred to as a continuous digester, which generally includes all or several of the processing stages through the completion of cooking at different levels of a single unit. When making dissolving pulp in a continuous digester, the pre-hydrolysis may be carried out at an upper portion of the continuous digester, and the cooking in a lower portion of the continuous digester.
Using a single-vessel continuous digester to make dissolving pulp may experience significant problems. For example, scaling or gunking eventually occurs due to the mixing of hydrolysate and kraft liquors. When this happens, the system must be shut down and cleaned. Also, it may be more difficult or impossible to separately recover hydrolysate as a by-product. In an attempt to overcome these problems, two-vessel continuous digesters have been developed, as exemplified for example by U.S. Pat. Nos. 4,436,586, 4,174,997 and 4,668,340. These systems are sometimes advertised as having the ability to maintain a cleaner separation between the acidic liquors in the pre-hydrolysis system and the caustic liquors in the kraft system. The two vessels of a continuous digester work in tandem to provide a continuous process whereby pre-steamed wood chips or similar matter is deposited into the top of the first vessel, are exposed to pre-hydrolysis as they pass downwardly in the first vessel until they are ultimately discharged from the bottom of that vessel, delivered to the top of the second vessel, and then exposed to a cooking process as they pass downwardly through the second vessel until they are ultimately discharged from the bottom of that vessel.
Many pulp facilities which focus on producing paper grade pulp employ single-vessel continuous digesters. These facilities are not well suited to making dissolving pulp. Using a single-vessel continuous digester to produce dissolving pulp would, as noted, result in scaling and gunking that would eventually require a shutdown of the system for cleaning.
There exists a need for pulp production system and method the permits greater flexibility, increased efficiency, or other benefits, and which generally may avoid the need to periodically shut down the facilities for cleaning resulting from reactions occurring during the manufacture of dissolving grade pulp.